Apparatus for detection of the presence of radio waves has traditionally sensed the presence of waves at only one frequency or a narrow band of frequencies. Where two or more substantially separated frequencies are to be sensed simultaneously, the conventional approach has been to merely "stack" replicas of the frequency-sensing apparatus together, each apparatus being designed for detection of one frequency (or a narrow band surrounding same) of the substantially separated frequencies. This requires duplication of the radiation-receiving horn, the frequency discriminating aperture and the associated detection circuitry for each frequency to be sensed.
U.S. Pat. No. 3,094,663 to Siegel teaches and claims apparatus for sensing the presence of continuous wave or modulated microwave radiation at a particular frequency using an antenna, a detector diode and modulator diode connected in series with opposite polarities on opposite sides of the antenna, with the cathode ends of the diodes being connected across a capacitance and being alternately rendered conducting and non-conducting. A low frequency signal, sensed across the capacitance in the presence of the high frequency signal whose presence is to be detected, is used to drive an audio and/or visual output device, for example at a tone corresponding to the lower frequency (imposed) signal. The higher frequency signal is not passed by the capacitance; and the threshold for signal pickoff at the detector is set so that the low frequency signal is not sensed at the output devices, in the absence of the higher frequency signal. The Siegel apparatus passes the signal developed at the capacitance directly to the output device and does not use an intermediate demodulator or low pass filter.
The use of an antenna connected to a modulator, which is in turn connected to or through a modulation generator, and to a detector, is well known in the prior art for receiving and preparing a radio wave signal. Examples of such use are found in Melabs Proposal JPL No. 1480 for For A Venus Space Probe Radiometer (Aug. 19, 1960, FIG. 4, p. 15) and in F. D. Drake, "Radio Astronomy Receivers", 19 Sky and Telescope, Nos. 1 and 2 Nov. and Dec. 1959, p. 13).